Optical fibers are commonly used in place of copper conductors to distribute signals. Applications include video distribution systems, data and voice communications, and sensor signal networks. Both multimode and single mode optical fiber systems are employed for such purposes. Conceptually, the distribution of optical power for any purpose requires that a source or combination of sources of optical power be divided among a number of accessible fiber optic connection points called ports. Each port serves as a separate source from which optical power may be fed into a separate optical fiber. Each fiber connected to a port may lead to a remote location for connection to user equipment. Generally, the source of optical power is modulated with information to be distributed among users.
Optical fiber systems obey essentially the same limitations as any communication medium in regard to the relationship between channel data capacity and signal to noise ratio. In any system there is an upper limit to the amount of optical power available, and there is noise. The lower the ratio between signal power and noise, the lower the channel capacity. In addition to the noise introduced by the electronic elements of the system, e.g., optical receiver amplifier noise, there may be optical noise. Among the common optical noise sources, optical power reflection is of considerable concern.
Optical power reflection occurs at any point in a fiber optic system where there is a discontinuity in the optical medium. Moreover, light traveling in an optical fiber reflects from the end of the optical fiber and travels back toward the source. Optical power reflected in this manner may reflect again when it arrives at the source point or other points in the system adding an unwanted noise component to the source signal. Optical power that is reflected back into a source can also corrupt the fundamental operation of the source. Lasers, e.g., are susceptible to unwanted optical power traveling back into the lasing cavity and often generate spurious optical signals as a result. To prevent back reflection, the end of an optical fiber, e.g., the end accessible at an unused port, must be terminated properly.
Another concern relates to the radiation of optical power from unused optical ports. As optical power sources become more powerful, there is an increasing risk of eye damage to personnel exposed to the open outputs of unused ports.
There may be hundreds of such ports co-located on distribution panels. On the other hand, fiber cables may run from a central distribution point to user access ports mounted, e.g., in a receptacle on the wall of the user's office. The connectors used as ports are usually expensive and subject to failure if exposed to abuse or dirt. Unused optical ports must, therefore, be protected from inadvertent exposure to contamination, contact with tools, etc.
To prevent back-reflections or hazardous radiation from unused ports, according to prior methods, a mating connector was fitted with a length of optical fiber, often called a pigtail, that extended outwardly from the connector. That pigtail fiber was terminated with a means that absorbed optical power. Thus, when the terminated pigtail fiber was connected to the unused port, optical power was passed through the mating connector into an absorptive means via the pigtail and was neither back-reflected nor radiated into the user's environment. The presence of the terminating connector also protected the port connector from damage.
While this means of terminating unused ports satisfies the basic requirements, the terminated pigtail fiber itself is subject to breakage. The pigtail fiber may be broken inside several layers of jacketing materials with no external indication of the flaw. The flaw reflects light back into the system with the potential of creating problems. Such flaws can be very difficult to locate. Additionally, a distribution panel may become cluttered with pigtails impairing the ability to identify unused ports.
Prior means of terminating the pigtail fiber consist of preparing the terminate end of the fiber so that it does not back-reflect light. Typically, to ensure minimum back-reflection from the end of an optical fiber it has been taught that the fiber end should be polished at angle relative to the longitudinal axis of the fiber. An angle sufficient that light travelling parallel to the axis of the fiber is internally reflected back into the fiber at an angle beyond the internal critical angle of the fiber ensures that any back-reflections are attenuated. Polishing is a time consuming operation adding considerable cost to the fabrication of terminating means.
Alternatively, it has also been suggested that crushing the end of the fiber in combination with a coating of light absorbing material applied to the crushed end is a sufficient means to satisfactorily eliminate back-reflected light. Repeated experimentation in an actual manufacturing environment has proven this prior teaching to be inaccurate. Merely crushing the end of a fiber and coating it with an appropriate material yields widely varying back-reflection levels. Moreover, the results obtained by simply crushing the end of the fiber and coating it with a composite material are not reliable nor repeatable enough for a manufacturing process.
Considering these several weaknesses of prior connector terminating means, a better approach is desirable.